About 25 years ago, a new bridge structural design using prestressed concrete was introduced. In recent years, the use of prestressed concrete bridge beams has been widespread and such design now incorporates a variety of structural configurations. Prestressed concrete bridge structural members are of two general types, pretensioned and post-tensioned. Current pretensioned construction designs usually consist of 7-wire strands, on the order of 1/2-inch in diameter, arranged in a matrix on 2-inch centers and the strands are tensioned prior to pouring the concrete. Beams with pretensioned members are usually made at a plant site because of the special fabrication facilities and tooling required. In the case of the post-tensioned configuration, ducts, usually metal, are placed in a specified location and configuration before the cement is poured; subsequently, the reinforcing strand, rod, or bar is inserted and tensioned, usually at the bridge site, and grouting material is introduced to fill the space between the reinforcing member and the duct.
The load-carrying capability of prestressed bridge structural members is directly dependent upon the strength of the steel reinforcement rods, bars, or strands; hence, the integrity of this steel is of primary concern and is influenced by one or more of the following factors:
(1) Quality of manufactured reinforcement material--governed by dimensional tolerances, strength, ductility, metallurgical type flaws such as voids or impurities, and mechanical damage such as nicks, gouges, etc. PA1 (2) Corrosion deterioration as a result of field environment. PA1 (3) Fracture failure as a result of over-stress (caused by loss of section due to corrrosion deterioration) or by impact loading (as a result of construction or vehicular impact). PA1 (4) Loss of bond between steel and concrete associated with corrosion of post-tensioned members due to voids in duct grouting collecting moisture.
In recent years, there is conclusive evidence that deterioration of the steel as a result of corrosion occurs; furthermore, such deterioration critically affects the structural strength. Currently used inspection procedures rely heavily on rust staining, cracking, and spalling of the concrete as an indicator that a problem exists in the reinforcing steel.
However, deterioration and even fracture of the reinforcing member can occur without being preceded by visual evidence on the external surfaces of the concrete members. For example, an elevated highway in a large city supported on 192 beams presently has more than 21 bars suspected of being fractured. Four such fractures have been confirmed. In this case, the presence of corroded and fractured post-tensioning bars was determined only from i) the projection of one end of the bars beyond the end of the beam during a visual inspection, ii) the loud noise made by one of the bars when it broke, which was heard by people in the area who reported it to the State, and iii) a broken bar which extended far beyond the end of the beam it was intended to reinforce, thereby interrupting traffic. There are no cracks or significant rust stains visible on the exterior surfaces of these particular beams.
From an overall point of view the problem is extremely broad because there is a wide variety of structural designs and the mechanisms contributing to the decrease or loss of structural integrity are complicated.